I purchased an SFX64 adaptor a while back for my PAL N64 and finally purchased an NTSC cart to try in it (in this instance California Speed).

However, it doesn't appear to work in the adaptor, just producing a black screen. My PAL N64 games appear to work fine through the adaptor.

So I was wondering if I have a broken adaptor or whether some (I.e later) N64 console models/releases block this particular or all such adaptors? Or have I just happened to pick an adaptor and game that for some reason, aren't compatible with each other?

I'm not sure how good your adaptor is, yet would recommend the N64 Passport Plus III as the very best one, though certainly not the cheapest. I bought mine when pursuing Ogre Battle, an NTSC game not available in Europe. This game is rumored to resist to most adaptors, yet no problems through the Passport Plus III. It works and memorizes code combinations, and I think you can access to the whole NTSC library this way, the best alternative to an actual NTSC system. Good luck!

Indeed, I since did a bit of research and believe I have a SFX64 v1 and that there were three versions of the SFX64.

I have a similar adaptor for the SNES that co-incidentally doesn't seem to have any issue with the non-pal games I've tried in it and hence I never really gave any consideration to the idea that the adaptor's inbuilt software may or may not be compatible with all games outside of exceptional circumstances (i.e like an emulator will probably never be 100% faithful to what it emulates).

I'm familiar with the SNES adaptors as well, the best ones will read almost everything, yet they did update US and Japanese protections over the years. There are two differerent runs of the US version of Super Mario RPG for example with two different kinds of protections, one supposedly being easier to run than the other. And then, you have the 50/60 hertz differences, some games you'll run via adaptors being prone to slowdowns or bugs sometimes. A minority though, but it's there. While it's possible to mod PAL systems so that they accept everything, bugs and slowdowns still persist in some cases. As much as possible, it's best to have an NTSC system on top of your PAL.